Spectrally resolved fast monitor

ABSTRACT

A method and apparatus for monitoring spectral tilt uses an arrayed waveguide grating (AWG) to separate a multiplexed optical signal having a plurality of wavelength channels into a plurality of sub-bands, where each sub-band spans a different wavelength range and includes more than one wavelength channel. A photodetector array is provided to measure the optical power in each of the sub-bands, while control electronics calculate spectral tilt of the multiplexed optical signal using the measured optical power in each of the sub-bands. The spectral tilt monitor in accordance with the instant invention provides spectral resolution, increased monitoring speeds, and decreased manufacturing costs.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/616,353 filed Oct. 6, 2004, the contents of which are incorporated byreference herein.

MICROFICHE APPENDIX

Not Applicable.

TECHNICAL FIELD

The present application relates generally to monitoring WDM opticalsignals, and in particular, to a method and apparatus for monitoringtilt in a WDM system having an optical amplifier.

BACKGROUND OF THE INVENTION

In wavelength division multiplexed (WDM) systems, multiple opticalchannels, each at a different wavelength, are multiplexed andsimultaneously transmitted through a single optical fiber. These opticalsignals need to be amplified (e.g., every 80 to 120 km) to compensatefor optical power loss in the optical fiber.

Optical amplifiers utilizing rare earth-doped fiber amplifiers (e.g.,EDFAs) in conjunction with optical pump provide optical power gainrequired to amplify all wavelengths simultaneously, thus lowering thecost of per channel amplification. Problems arise, however, when opticalamplifiers do not provide uniform optical power gain to all of thetransmitted wavelength channels.

Optical power variations between wavelengths, or spectral distortion,arises directly from optical amplifier gain (e.g., a non-uniform gainprofile), and is further distorted by accumulated distortion.Furthermore, shorter wavelengths act as additional power pumps causinglonger wavelength to experience additional gain known as StimulatedRaman Scattering (SRS). As a result, spectral tilt occurs with positiveslope that continues to increase along the amplifiers chain.

In order to accurately correct and/or control spectral tilt in timelyfashion, the tilt needs to be monitored and its slope accuratelymeasured in real time. Conventional monitoring approaches use spectrumanalyzers, wherein the spectrum is scanned or demultiplexed intoindividual wavelength channels where the optical power of each of thedemultiplexed wavelength channels is measured separately. While spectrumanalyzers are precise, they are also expensive and relatively largedevices. Furthermore, while these monitors are accurate for measuringtilt caused by steady-state signal power variations, they are generallytoo slow for monitoring tilt caused by fast provision, optical channelrestoration and optical power transients caused by fiber cut orequipment failures. All these events can cause significant spectraldistortions and positive or negative tilt in a time scale of less thanmicrosecond, rendering prior monitoring techniques ineffective inmitigating the negative impact of services.

Monitors for measuring spectral tilt caused by these fast transients aretypically single-point monitors that only measure the total power of theoptical signal. More specifically, these monitors estimate the spectraltilt using the linear relationship between SRS-induced spectral tilt andtotal power of the optical signal. These fast monitors, however, lackthe spectral resolution and the determination whether the tilt has apositive or negative slope, information necessary to take correctiveactive.

It is an object of this invention to provide a spectrally resolved fastmonitor for measuring spectral tilt.

It is a further object of the instant invention to provide a spectrallyresolved fast monitor for measuring spectral tilt that is relativelycompact and low cost.

It is a further object of the instant invention to provide fast feedbackwith accurate data to drive a tilt correcting device and/or drive theoptical pump to adjust its optical pump power to a higher or lowermagnitude as required.

SUMMARY OF THE INVENTION

The instant invention relates to a method and apparatus for monitoringspectral tilt wherein the optical signal is separated into a pluralityof sub-bands, each sub-band spanning a different wavelength range, thenumber of sub-bands being less than the number of wavelength channels.Spectral tilt is calculated using the optical power measured in each ofthe sub-bands.

According to one embodiment, the optical signal is separated into theplurality of sub-bands using a plurality of thin-film-filters (TFFs).According to another embodiment, the optical signal is separated intothe plurality of sub-bands using an arrayed waveguide grating (AWG).

Advantageously, the spectral tilt monitor is designed to providespectral resolution, to increase monitoring speeds, and to decreasemanufacturing costs. Moreover, it can be designed to provide a flattransmission over an arbitrarily wide and sub-band passband with noinherent design loss, while still being able to separate bands ofchannels at the resolution of the channel spacing with very highisolation and no requirement for dead or skipped channels.

In accordance with one aspect of the instant invention there is provideda method of monitoring spectral tilt comprising: passing a multiplexedoptical signal having a plurality of wavelength channels through aspectral tilt monitor such that an arrayed waveguide grating in thespectral tilt monitor separates the multiplexed optical signal into aplurality of sub-bands, each sub-band including more than one wavelengthchannel from the plurality of wavelength channels; measuring the opticalpower in each of the sub-bands; and, calculating spectral tilt of themultiplexed optical signal using the measured optical power in each ofthe sub-bands.

In accordance with one aspect of the instant invention there is furtherprovided a spectral tilt monitor comprising: an arrayed waveguidegrating for separating a multiplexed optical signal having a pluralityof wavelength channels into a plurality of sub-bands, each sub-bandincluding more than one wavelength channel from the plurality ofwavelength channels.

According to one embodiment the number of wavelength channels in theplurality of wavelength channels is n, and the arrayed waveguide gratingincludes: an optical waveguide for transmitting the multiplexed opticalsignal; a waveguide array having a first end optically coupled to afirst slab waveguide and a second end optically coupled to a second slabwaveguide, the first slab waveguide for receiving the multiplexedoptical signal from the optical waveguide, the second slab waveguide forfocussing n demultiplexed sub-signals of the multiplexed optical signalto n separate locations; and a plurality of multi-mode output waveguidesoptically coupled to the second slab waveguide, each multi-mode outputwaveguide having a width selected to collect a plurality of thedemultiplexed sub-signals and provide one of the sub-bands.

In accordance with one aspect of the instant invention there is furtherprovided an arrayed waveguide grating comprising: an optical waveguidefor transmitting a multiplexed optical signal having n wavelengthchannels; a waveguide array having a first end optically coupled to afirst slab waveguide and a second end optically coupled to a second slabwaveguide, the first slab waveguide for receiving the multiplexedoptical signal from the optical waveguide, the second slab waveguide forfocussing n demultiplexed sub-signals of the multiplexed optical signalto n separate locations; and, a plurality of multi-mode outputwaveguides optically coupled to the second slab waveguide, eachmulti-mode output waveguide having a width selected to collect aplurality of the demultiplexed sub-signals.

In accordance with another aspect of the instant invention there isprovided a spectral tilt monitor comprising: an input for providing amultiplexed optical signal having a plurality of wavelength channels; aplurality of thin-film-filters optically coupled to the input forseparating the multiplexed optical signal into a plurality of sub-bands,each sub-band including more than one wavelength channel from theplurality of wavelength channels; a photodetector for measuring theoptical power in each of the sub-bands; and control electronics forcalculating spectral tilt of the multiplexed optical signal using themeasured optical power in each of the sub-bands.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will becomeapparent from the following detailed description, taken in combinationwith the appended drawings, in which:

FIG. 1 is a schematic diagram of a node in a WDM system having aspectral tilt monitor in accordance with one embodiment of the instantinvention;

FIG. 2A shows the spectral tilt calculated with a high channel count andan even channel distribution;

FIG. 2B shows the spectral tilt calculated when one sub-band is fullyloaded and the others are not loaded;

FIG. 2C shows the spectral tilt calculated when one sub-band is fullyloaded and the others are partially loaded;

FIG. 2D compares the spectral tilt calculated in FIG. 2C with thatcalculated with a normalized band power;

FIG. 3 is a flow chart illustrating one embodiment of an algorithm forcalculating spectral tilt;

FIG. 4A illustrates the spectral tilt calculated with the algorithmdiscussed in FIG. 3 when the 40 channel wavelength band is fully loaded;

FIG. 4B illustrates the spectral tilt calculated with the algorithmdiscussed in FIG. 3 when 16 of the 40 channel wavelengths are loaded;

FIG. 4C illustrates the spectral tilt calculated with the algorithmdiscussed in FIG. 3 in an extreme tilt case;

FIG. 4D illustrates the spectral tilt calculated with the algorithmdiscussed in FIG. 3 in an extreme tilt case;

FIG. 5 is a schematic diagram of an demultiplexer suitable for use inthe spectral tilt monitor shown in FIG. 1;

FIG. 6A is a schematic diagram of another demultiplexer based on a AWGthat is suitable for use in the spectral tilt monitor shown in FIG. 1;

FIG. 6B is a schematic diagram illustrating the position and width ofstandard 40 channel outputs relative to the multi-mode waveguides;

FIG. 6C shows a comparison of the achievable band separation filteringusing standard AWG flattop techniques (top), a scaled version (middle)and the instant invention (bottom);

FIG. 6D is a schematic diagram of another demultiplexer based on a AWGthat is suitable for use in the spectral tilt monitor shown in FIG. 1;

FIG. 7 is a schematic diagram of a bi-directional node in a WDM systemhaving a spectral tilt monitor in accordance with one embodiment of theinstant invention; and,

FIG. 8 is a schematic diagram of a chip having four AWG with multi-modalouput waveguides.

It will be noted that throughout the appended drawings, like featuresare identified by like reference numerals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1 there is shown a schematic diagram of a node 10 in aWDM system. The node 10 includes an optical amplifier 20, a spectraltilt monitor 30, a spectral tilt compensator 40, and a tap 50. Otherelements, for example dispersion compensating modules (DCMs) (notshown), are optionally included.

The optical amplifier 20 amplifies WDM optical signals transmitted intothe node 10. One example of a suitable optical amplifier is a rare-earthdoped fiber amplifier, such as an erbrium-doped fiber amplifier (EDFA).Of course, other optical amplifiers, such as a Raman amplifier or EDFARaman hybrid, are also possible. The optical amplifier 20 boosts thepower level of all of the optical signals carried in the wavelengthchannels at the same time, while simultaneously introducing spectraltilt and/or intensifying spectral tilt introduced upstream from theamplifier 20.

The spectral tilt monitor 30 monitors and/or measures the spectral tiltin the amplified light. In accordance with the instant invention, thespectral tilt monitor 30 includes a band demultiplexer 31 for separatingthe WDM optical signal into a plurality of sub-bands (i.e., wavelengthbands). Each sub-band spans a different wavelength range and has a knownmaximum number of wavelength channels. The bandwidth of each sub-band isthe same or different from adjacent sub-band(s), but does not over-lapother sub-bands. Each sub-band is either fully loaded, not loaded, orpartially loaded. The spectral tilt-monitor 30 also includes a detector39 for measuring the total optical power of each sub-band and controlelectronics 37 for calculating the spectral tilt based on the totaloptical power measured in each sub-band.

The spectral tilt compensator 40 receives a control signal from thecontrol electronics 37 and compensates for the spectral tilt. Someexamples of suitable spectral tilt compensators include gain flatteningfilters (GFFs), variable optical attenuators (VOAs), and/or dynamic gainequalizers (DGEs). Alternatively, the spectral tilt compensator isprovided as part of the optical amplifier (i.e. spectral tilt may becompensated by adjusting the operating conditions of the opticalamplifier) and/or as part of a 2-stage optical amplifier.

The tap 50, taps a relatively small portion (e.g., 5%) of the amplifiedoptical signal and redirects it to the spectral tilt monitor 30, whilethe remaining part of the amplified optical signal is transmitted to thespectral tilt compensator 40. One example of a suitable tap is a 5/95coupler tap.

Advantageously, the spectral tilt monitor in accordance with the instantinvention provides increased monitoring speed and decreasedmanufacturing costs relative to a spectrum analyzer (e.g., there are fewphotodetectors required). Furthermore, the spectral tilt monitor inaccordance with the instant invention provides increased spectralresolution relative to a single power measurement.

Notably, this increased spectral resolution provides for a more accuratecalculated spectral tilt, particularly when wavelength channels areadded and/or dropped from the WDM system. For example, consider theexperimental and simulation results illustrated in FIGS. 2A,2B, and 2C.For exemplary purposes, the spectral tilt is calculated through a linearfitting of four data points, each data point corresponding to the totaloptical power in one of four sub-bands. In FIG. 2A, wherein the opticalsignal has a high channel count and an even channel distribution, theplotted spectral tilt is shown to be very accurate. In FIG. 2B, whereinone sub-band is fully loaded and the others are not loaded, the plottedspectral tilt is poor. In FIG. 2C, wherein one sub-band is fully loadedand the others are partially loaded, the plotted spectral tilt is alsoerroneous. In the latter instances, however, it is possible to use thespectral resolution to normalize the optical power per sub-band and toincrease the accuracy of the calculated spectral tilt. For example,consider FIG. 2 d, which compares the spectral tilt obtained from themeasured band power (band power) and a normalized band power(normalized). The normalized band powers are calculated using a knownchannel loading and/or an estimated channel loading.

Referring to FIG. 3, there is shown one example of an algorithm whereinthe total power in each band is essentially normalized by convertingoptical power to units of dB to determine relative power gain or lossaccording to the channel loading. In the first step 60, the spectraltilt monitor measures the optical power in each of the sub-bands usingthe plurality of photodetectors. Next, the total optical power from allof the wavelength channels is calculated as the sum of the measuredoptical powers 62. In step 64, the total optical power is compared to apreviously calculated value. If the total optical power is relativelyconstant, then step 60 is repeated. If the total optical power issignificantly different from the previously calculated value, thenspectral tilt is calculated according to steps 68-72. More specifically,spectral tilt is calculated as follows: an estimated average power perchannel is calculated for each sub-band (68); the number of loadedchannels per sub-band is determined by dividing the measured opticalpower per sub-band by the estimated average power per channel (70), andfinally, using the number of loaded channels determined in step 70 andthe measured optical powers in step 60, a normalized band power in unitsof dB is calculated (72). These normalized band powers are plotted toyield the new spectral tilt.

In step 67, the spectral tilt is estimated using the total signal powerin the fiber, attenuation of the fiber, dispersion, the fiber type, andthe fact that SRS induced slope depends on shorter wavelength loading.Accordingly, the pre-stored parameters include: total power and numberof channels in a fully loaded system, fiber span length, input power,fiber type, and a plot of SRS induced tilt vs. fiber length.

In step 68, the average power per channel is calculated using thefollowing equation:

${P(j)} = {P_{Ave} + {\frac{\mathbb{d}P}{\mathbb{d}f}\left( {f_{j} - {\text{<}f\text{>}}} \right)}}$where P(j) is equal to the average power per channel in band number j,P_(Avg) is equal to the average power per channel obtained from sum ofthe normalized powers from the photodetectors divided by the totalnumber of channels in the fully loaded system, f_(j) is the centerfrequency of the j^(th) band, and <f> is the average center frequencyover all the bands. Notably, this equation assumes that the channeldistribution is linear.

FIGS. 4 a-4 d illustrate various spectral tilts simulated using theabove algorithm. In these simulations, the optical signal is assumed tobe a standard 40 channel WDM with 100 GHz spacing. In FIG. 4A, whereinthe 40 channel wavelength band is fully loaded, the 1.67 dB spectraltilt is calculated to be 1.48 dB after the first iteration and 1.67 dBafter the second iteration. In FIG. 4B, wherein 16 of the 40 channelsare loaded, the 1.67 dB spectral tilt is calculated to be 0.95 dB afterthe first iteration and 1.80 after the second iteration. In FIGS. 4C and4D, wherein the spectral tilt of the fully loaded 40 channel wavelengthband is non-linear, the −2.68 dB spectral tilt is impressively estimatedto be −2.15 dB after the fourth iteration and −2.68 after the fifthiteration.

Referring to FIG. 5, there is shown one embodiment of the banddemultiplexer 31 described in FIG. 1. The demultiplexer includes fourthin-film-filters (TFFs) 15 a-d, each of which is shown coupled to adifferent photodetector 17 a-d. One example of a suitable photodetectoris an edge mounted photodiode. Optionally, the four photodetectors areprovided as an array to reduce manufacturing costs.

In operation, a 40 channel WDM optical signal is launched into thedemultiplexer where it is directed to a first TFF 15 a. The first filter15 a passes a first portion (i.e., a first sub-band) of the opticalsignal to the first photodetector 17 a, where the optical power ismeasured, and a second portion to the next TFF 15 b. The second filter15 b passes a first portion (i.e., a second sub-band) of the filteredoptical signal to the photodetector 17 b, where the optical power ismeasured, and a second portion to the next TFF 15 c. The third filter 15c passes a first portion (i.e., a third sub-band) of the twice filteredoptical signal to the photodetector 17 c, where the optical power ismeasured, and a second portion to the next TFF 15 d. The filter 15 dpasses the remaining optical signal (i.e., the fourth and remainingsub-band) to the fourth photodetector 17 d. The spectral tilt iscalculated using the optical power measured for each of the sub-bands(e.g., using the algorithm described with respect to FIG. 3).Optionally, the spectral tilt is calculated with an algorithm (notshown) that compensates for channels between sub-bands that aresuppressed by the TTFs.

Advantageously, using a spectral tilt monitor based on TFFs providesincreased spectral resolution, provides feedback about whether the tilthas a positive or negative slope, is fast, and is reliable. With regardto monitoring speed, the spectral tilt monitor in accordance with theinstant invention has been found to correct tilt using a VOA and/or GFFin less than about one microsecond. Although VOAs and GFFs are lessaccurate than some compensating devices, it has been found that thecombination of the TTF based spectral tilt monitor with a fast tiltcompensator (using VOAs and GFFs) provides close to optimal end-to-endperformance.

Referring to FIGS. 6A and 6B, there is shown another embodiment of theband demultiplexer 31 described with respect to FIG. 1. Thedemultiplexer is based on an AWG that includes a single, single-modeinput waveguide 32, a first slab waveguide 34, a waveguide array 35having a plurality of single-mode waveguides with different lengths, asecond slab waveguide 36, and four wide, highly multi-mode outputwaveguides 38 a-d, all of which are disposed on a single substrate 33.For illustrative purposes, the AWG 31 is based on a standard 40 channel100 GHz AWG. Also shown are four photodetectors 39 a-d, which arecoupled to the substrate 33 such that each of the photodetectors 39 a-dis positioned at the end of a different multi-mode waveguide 38 a-d.Preferably, each photodetector 39 a-d has a width sufficiently large tocollect all the light from the corresponding multi-mode output waveguide38 a-d. This criteria is met, for example, if each photodetector has awidth greater than the width of the output of the correspondingmulti-mode waveguide. One example of a suitable photodetector is an edgemounted photodetector. Optionally, the four photodetectors are providedas an array to reduce manufacturing costs.

In operation, a 40 channel WDM optical signal is launched into thesingle-mode input waveguide 32 where it is passed through the first slabwaveguide 34 and directed into the waveguide array 35. Since theplurality of waveguides in the array have different lengths, thedifferent portions of optical signal propagating through differentwaveguides will have different phases and interference will occur. Thisinterference results in the demultiplexing of the optical signal, thedemultiplexed components of which are imaged on the outside edge of thesecond slab waveguide 36. In a conventional AWG, which is well known inthe art and not discussed further, each of the imaged demultiplexedcomponents is collected by a different waveguide. In the AWG shown inFIG. 6A, the multi-mode optical waveguides 38 a-d collect small groupsof the imaged demultiplexed components (i.e., small groups of adjacentwavelength channels). As illustrated in FIG. 6B, each of the fourmulti-mode output waveguides 38 a-d is sufficiently wide to collect 10adjacent wavelength channels (i.e., 38 a collects λ₁-λ₁₀, 38 b collectsλ₁₁-λ₂₀, 38 c collects λ₂₁-λ₃₀, and 38 d collects λ ₃₁-λ₄₀). The opticalpower of these sub-bands is measured with the photodetectors 39 a-d andthe spectral tilt is calculated (e.g., using the algorithm describedwith respect to FIG. 3).

Advantageously, the AWG described above provides increased spectralresolution, provides feedback about whether the tilt has a positive ornegative slope, provides a wide flattop line shape for each sub-bandwith no design loss penalty, achieves no-gap operation, is fast, isaccurate, is reliable, and is easily integrated with other components toform a compact device. Several of these advantages are realized becausethe AWG possesses sufficient resolution to resolve the number ofchannels in the optical signal (i.e., in this embodiment 40), but thencompromises resolution by grouping the channels into sub-bands.

Advantageously, the AWG described above provides increased spectralresolution, provides feedback about whether the tilt has a positive ornegative slope, provides a wide flattop line shape for each sub-bandwith no design loss penalty, achieves no-gap operation, is fast, isaccurate, is reliable, and is easily integrated with other components toform a compact device. Several of these advantages are realized becausethe AWG possesses sufficient resolution to resolve the number ofchannels in the optical signal (i.e., in this embodiment 40), but thencompromises resolution by grouping the channels into sub-bands.

With regard to the wide flattop shape, one is directed to FIG. 6C, whichshows the achievable band separation filtering of a standard AWG flattopdesign compared to an AWG having multi-mode output waveguides. The topdiagram in FIG. 6C shows a conventional flattop AWG spectrum designed todemultiplex an individual channel. The spectrum exhibits ripple acrossthe passband and an inherent design loss. One example of such a flattopdesign is discussed in U.S. Pat. No. 5,412,744, hereby incorporated byreference. The middle diagram of FIG. 6C shows the result of simplyscaling the spectrum of the top diagram to cover multiple channels in aband (the grey boxes denote the grouping of individual channel passbandsinto a sub-band). The resulting spectrum exhibits large crosstalk overthe adjacent sub-bands. In contrast, the bottom diagram shows thespectrum of the instant invention which has a wide, flat passband, withno inherent passband loss, and very low crosstalk across adjacentsub-bands.

With regard to achieving no-gap operation, the AWG illustrated in FIGS.6 a,b is able to minimize the space between adjacent sub-bands to lessthan one channel spacing in a 100 GHz system. In other words, AWGtechnology has the potential to provide a 10-skip-0 gap. Minimizing thegap between sub-bands is important because channels in the gaps will beneglected or partially neglected in the power measurements, and thuswill have a direct impact on the monitoring accuracy. Notably, TFFsand/or other demultiplexing techniques are not able to provide thislevel of accuracy.

With regard to monitoring speed, the spectral tilt monitor in accordancewith the instant invention has been found to correct tilt using a VOAand/or GFF in less than about one microsecond.

In the embodiment described with regards to FIGS. 6 a,b, the AWG iseither temperature sensitive or a thermal. In the prior art, AWGs aregenerally fabricated to be a thermal to prevent the wavelength channelsfrom shifting with changes in temperature. In the AWG of the instantinvention, however, temperature correction and/or stabilization is notalways necessary. For example, since the sub-band bandwidth is wide andthe channels discrete, the change in optical power of each sub-band willlargely depend only on its edge channels. In other words, the totalimpact will be dependent on the band population distribution. In a fullypopulated band, negligible slope and/or power errors have been found forthe full −5 to 65 operating range.

Referring to FIG. 6D, there is shown another embodiment of the banddemultiplexer 31 described with respect to FIG. 1. The demultiplexer isbased on an AWG 131 that includes a single, single-mode input waveguide132, a first slab waveguide 134, a waveguide array 135 having aplurality of single-mode waveguides with different lengths, a secondslab waveguide 136, and four multi-mode output waveguides 138 a-d, allof which are disposed on a single substrate 133. For illustrativepurposes, the AWG 131 is based on a standard 40 channel 100 GHz AWG. Aheater 129 is coupled to the substrate so as to uniformly heat thewaveguide array 135. Four photodetectors 139 a-d, which are also coupledto the substrate 133, are each positioned at the end of a differentmulti-mode waveguide 138 a-d. Preferably, each photodetector 139 a-d hasa width sufficiently large to collect all the light from thecorresponding multi-mode output waveguide 138 a-d. This criteria is met,for example, if the photodetector has a width greater than the width ofthe output of the corresponding multi-mode waveguide; One example of asuitable photodetector is an edge mounted photodetector. Optionally, thephotodetectors 138 a-d are provided as an array to reduce manufacturingcosts.

Notably, this embodiment takes advantage of the fact that the centerwavelength of each of the sub-bands is tunable with changes intemperature. This temperature sensitivity is used to further improve thespectral resolution.

In operation, a 40 channel WDM optical signal is launched into thesingle-mode input waveguide 132 where it is passed through the firstslab waveguide 134 and directed into the waveguide array 135. Since theplurality of waveguides in the array have different lengths, thedifferent portions of optical signal propagating through differentwaveguides will have different phases and interference will occur. Thisinterference results in the demultiplexing of the optical signal, thedemultiplexed components of which are imaged on the outside edge of thesecond slab waveguide 136. Each of the four multi-mode output waveguides138 a-d is fabricated to be sufficiently wide to collect 10 adjacentwavelength channels. The optical power of each of the four sub-bands ismeasured with a different photodetector (i.e., one of 139 a-d). Upon afirst activation of the heater 129, the temperature is increased. Thisincrease in temperature shifts the center wavelengths of the lightcollected in each of the sub-bands to a higher wavelength value. Sincethe edge channels of the sub-bands change, this results in four newoptical power measured readings. Upon a second activation of the heater129, the temperature is decreased. This decrease in temperature shiftsthe center wavelengths of the light collected in each of the sub-bandsto a lower wavelength value. Since the edge channels of the sub-bandschange again, this results in an additional four new optical powermeasured readings. The spectral tilt is then plotted as a least squarefit using the twelve optical power measurements. Advantageously, thisconfiguration results in an increased spectral resolution for the samenumber of photodetectors.

The spectral tilt monitor 30 illustrated in FIG. 1 is depicted in afeedback configuration for illustrative purposes. In addition, oralternatively, it is possible for the spectral tilt monitor 30 to beused in a feed-forward configuration. In fact, in many WDM systems it isadvantageous to monitor spectral tilt both upstream and downstream ofthe optical amplifier 20.

Referring to FIG. 7, there is shown a schematic diagram of a node in abi-directional fiber link. The node 100 is shown to include first 120 aand second 120 b optical amplifiers, a spectral tilt monitor 130, andvarious taps 150 a-d.

The first 120 a and second 120 b optical amplifiers amplify WDM opticalsignals transmitted into the node 100. One example of a suitable opticalamplifier is a rare-earth doped fiber amplifier, such as anerbrium-doped fiber amplifier (EDFA). Of course, other opticalamplifiers, such as a Raman amplifier or Raman EDFA hybrid, are alsopossible. The optical amplifier 120 boosts the power level of all thewavelength channels in the WDM optical signal(s) at the same time, whilesimultaneously introducing spectral tilt and/or intensifying spectraltilt introduced upstream from the amplifiers 120 a/120 b.

The spectral tilt monitor 130 monitors and/or measures the spectral tiltupstream and downstream of the optical amplifiers 120 a/120 b. Inaccordance with the instant invention, the spectral tilt monitor 130includes four AWGs, each AWG for separating the WDM optical signal(s)into four sub-bands, each sub-band having a different wavelength rangeand a known maximum number of wavelength channels. The bandwidth of eachsub-band is the same or different from adjacent sub-band(s). Eachsub-band is either fully loaded, not loaded, or partially loaded. Thespectral tilt-monitor 130 also includes a plurality of photo-detectors139 for measuring the total optical power of each sub-band and controlelectronics 137 for calculating the spectral tilt based on the totaloptical power in each sub-band. Optionally, a temperature controller(not shown) is provided to tune the AWG.

A spectral tilt compensator (not shown) receives a control signal fromthe spectral tilt monitor 130 and compensates for the spectral tilt.Some examples of suitable spectral tilt compensators include gainflattening filters (GFFs), variable optical attenuators (VOAs), and/ordynamic gain equalizers (DGEs). Alternatively, the spectral tiltcompensator is the optical amplifier 120 a/120 b.

The taps 150 a-d, tap a relatively small portion (e.g., 10%) of theamplified optical signal(s) and redirect it to the spectral tilt monitor130, while the remaining part of the optical signal. One example of asuitable tap is a 10/90 coupler tap.

Advantageously, the spectral tilt monitor 130 integrates four AWGs,sixteen photodiodes, a temperature controller, and/or the photodiodebias circuitry, all on the same chip. Moreover, one set of controlelectronics is used to monitor and calculate the spectral tilt of eachof the four tapped signals. Accordingly, this configuration results in avery compact device and reduces the required number of components.Referring to FIG. 8, there is shown one configuration of a chip withfour AWGs that has been found to fit in a standard, fully qualified, AWGtype package.

The embodiments of the invention described above are intended to beexemplary only. For example, while the embodiments described above arebased on a standard 40 channel 100 GHz AWG with multi-mode outputwaveguides having a width sufficient to collect about 10 wavelengthchannels, thereby providing a means for monitoring the optical power infour different sub-bands, it is also possible to monitor the spectraltilt using more or fewer sub-bands in WMD systems with more or lesschannels. Furthermore, while the AWG with multi-mode output waveguidesis very valuable for use in the spectral tilt monitor, other uses arealso envisioned. The scope of the invention is therefore intended to belimited solely by the scope of the appended claims.

1. A method of monitoring spectral tilt comprising: passing amultiplexed optical signal having a plurality of wavelength channelsthrough a spectral tilt monitor such that an arrayed waveguide gratingin the spectral tilt monitor separates the multiplexed optical signalinto a plurality of sub-bands, each sub-band including more than onewavelength channel from the plurality of wavelength channels; measuringthe optical power in each of the sub-bands; and, calculating spectraltilt of the multiplexed optical signal using the measured optical powerin each of the sub-bands, wherein the arrayed waveguide gratingincludes: an optical waveguide for transmitting the multiplexed opticalsignal; a waveguide array having a first end optically coupled to afirst slab waveguide and a second end optically coupled to a second slabwaveguide, the first slab waveguide for receiving the multiplexedoptical signal from the optical waveguide, the second slab waveguide forproviding demultiplexed sub-signals of the multiplexed optical signal;and a plurality of multi-mode output waveguides optically coupled to thesecond slab waveguide, each multi-mode output waveguide having a widthselected to collect a plurality of the demultiplexed sub-signals andprovide one of the sub-bands.
 2. A method according to claim 1, whereincalculating the spectral tilt includes using an algorithm that estimatesan average channel power for each sub-band and a number of channels foreach sub-band.
 3. A method according to claim 1, comprising compensatingfor the spectral tilt using at least one of a variable opticalattenuator and a gain flattening filter.
 4. A method according to claim1, comprising heating the arrayed waveguide grating to tune the spectraltilt monitor.
 5. A spectral tilt monitor comprising: an arrayedwaveguide grating for separating a multiplexed optical signal having aplurality of wavelength channels into a plurality of sub-bands, eachsub-band including more than one wavelength channel from the pluralityof wavelength channels, wherein the number of wavelength channels in theplurality of wavelength channels is n, and wherein the arrayed waveguidegrating includes: an optical waveguide for transmitting the multiplexedoptical signal; a waveguide array having a first end optically coupledto a first slab waveguide and a second end optically coupled to a secondslab waveguide, the first slab waveguide for receiving the multiplexedoptical signal from the optical waveguide, the second slab waveguide forfocussing n demultiplexed sub-signals of the multiplexed optical signalto n separate locations; and a plurality of multi-mode output waveguidesoptically coupled to the second slab waveguide, each multi-mode outputwaveguide having a width selected to collect a plurality of thedemultiplexed sub-signals and provide one of the sub-bands.
 6. Aspectral tilt monitor according to claim 5, wherein the opticalwaveguide is a single mode waveguide.
 7. A spectral tilt monitoraccording to claim 5, comprising a photodetector optically coupled toeach multi-mode output waveguide for measuring the optical power in eachof the sub-bands.
 8. A spectral tilt monitor according to claim 7,comprising control electronics for calculating spectral tilt of themultiplexed optical signal using the measured optical power in each ofthe sub-bands.
 9. A spectral tilt monitor according to claim 7, whereinthe photodetectors are part of a photodetector array.
 10. A spectraltilt monitor according to claim 9, wherein the photodetector array,control electronics, and arrayed waveguide grating are integrated on asame substrate.
 11. A spectral tilt monitor according to claim 5,comprising a heating element for tuning the arrayed waveguide grating.12. A spectral tilt monitor according to claim 5, wherein each sub-bandin the plurality of sub-bands spans a different wavelength range.
 13. Aspectral tilt monitor according to claim 5, wherein the arrayedwaveguide grating has sufficient resolution to resolve the plurality ofwavelength channels.
 14. A spectral tilt monitor according to claim 5,comprising four arrayed waveguide gratings for monitoring abi-directional fiber link.
 15. A spectral tilt monitor according toclaim 14, wherein the four arrayed waveguide gratings, a photodetectorarray, and control electronics are all integrated on a same substrate.